Hydraulic molding presses are well-known and have been used for a variety of purposes including the hot pressing of adhesive or resin coated fibers, chips or sheets of wood, paper and the like to form boards, panels, plywood and other products of a similar nature.
The manufacture of wafer board panels from wood wafers and a heat curable resin is one such illustration. Numerous patents exist which refer to the manufacture of wafer board panels including patents to such inventors as James d'A. Clark and Robert Lambert. A useful description of the entire process is shown in U.S. Pat. No. 3,899,559.
Although early efforts at making panels such as plywood and wafer board in hydraulic molding presses involve the use of relatively small presses of conventional design, economic realities have dictated the use of larger and larger presses. As the size of the presses has increased, concern has been generated over the ability of presses of conventional design using circular pistons to achieve uniform pressing and proper thickness control.
In an effort to improve the operation of such processes, a hydraulic molding press was devised using an extremely large non-circular piston of approximately the same dimensions as the lower platen of the hydraulic press. This development is described in U.S. Pat. No. 2,875,797.
The use of a hydraulic molding press having a non-circular piston of substantially the same size and substantially the same dimensions as the lower press platen offers certain advantages in quality control with regard to the product (e.g. wafer board) being produced. On the other hand, such presses must have an effective seal surrounding the non-circular piston for preventing loss of the working fluid (e.g. water). Conventional seals intended for circular pistons have not proven suitable and special seals have been developed, largely on a trial and error basis.
Seal problems seem to have become more difficult as the size of the non-circular piston has been increased. For example, one single, non-circular piston press which is now in operation in the U.S.A. in the production of wafer board is approximately 8 feet wide by 28 feet long.
Since changing a seal on a press of this size is a time consuming task and involves a complete shutdown of manufacturing operations, there has been substantial incentive to develop seals capable of operating for extended periods of time without replacement.